ci: use default Rust workflow

[Nidhi-Gahlawat]

Summary

• replace .github/workflows/main.yml with the standard rust.yml workflow
• consolidate the Linux and no_std jobs into one canonical build matrix
• preserve stable, beta, nightly, MSRV, and no_std builds along with all existing test, documentation, benchmark, Miri, and fuzz commands
• retain the homu build finished aggregation signal used by current successful v2 runs

Validation

• actionlint .github/workflows/rust.yml
• git diff --check
• compared workflow commands, toolchains, targets, and triggers against upstream/v2
• Cargo commands were not run locally because Rust is not installed on the development machine; the commands themselves are unchanged

Closes #470

[alejandro-vaz]

this looks better than what we had

please remember that AI contributions are not allowed in any @servo repository as indicated in the contributing guidelines

thanks for contributing